In general, the use of LEDs for illumination purposes is known. A problem with LEDs is the power supply. For a LED to produce light, it requires a current to pass through it in one direction (from anode to cathode); current flow in the opposite direction is blocked. When driven with current having the correct direction, a voltage drop develops over the LED which is substantially independent of the LED current. Within margins, the LED current can be varied, and the light output will be substantially proportional to this current. When it is desirable to produce more light than one LED can generate, it is possible to combine multiple LEDs. The LEDs can be arranged in a series arrangement, which would require a higher voltage drop at the same current, or the LEDs can be arranged in a parallel arrangement, which requires more current at the same voltage drop. Thus, the costs of power supply increase. Combinations of series arrangement and parallel arrangement are also possible.
A relatively simple and cheap way of powering a plurality of LEDs is to connect all LEDs in series and to connect this string to AC power mains, having a current limiting resistor in series. Obviously, the LEDs can only produce light during one half of the AC current period. For also producing light during the second half of the AC current period, a second string of LEDs may be connected in the opposite direction, or a full bridge rectifier may be applied so that each LED produced light during both current half periods.
A problem when powering a LED or a string of LEDs from an AC source is that the supply voltage varies with time. FIG. 1 is a graph showing voltage (vertical axis) as a function of time (horizontal axis). A horizontal dotted line 11 represents the required voltage drop, also indicated as forward voltage, over a string of LEDs. Curve 12 represents rectified AC voltage. Between times t1 and t2, the supply voltage is higher than the required voltage drop, and the LEDs pass a current (curve 13) and light is generated. The difference between supply voltage and voltage drop is accommodated by the series resistor, and involves loss of energy by dissipation in the resistor. Between times t2 and t3, the supply voltage is lower than the required voltage drop: the LEDs can not pass current and can not generate light. Thus, the LEDs are not continuously ON but are actually switched ON/OFF at a frequency of twice the AC frequency, leading to noticeable flicker, and at a duty cycle (t2-t1)/(t3-t1) that is influenced by the voltage amplitude of the power supply in relation to the required voltage drop over the LEDs, which depends on the number of LEDs arranged in series. It should be clear that the duty cycle can be increased by increasing the voltage amplitude, but then also the power dissipated in the resistor will increase.